The quality of water in reservoirs is of great importance to society. Many publications have been written that deal with water quality of reservoirs, such as the US Army Corps of Engineers Engineer Manual 1110-2-1201, entitled “Reservoir Water Quality Analysis”, from which background information herein has been taken.
“Water quality”, as defined herein, includes the physical, chemical, and biological characteristics of water and the abiotic and biotic interrelationships.
Any reservoir or stream system is coupled with its watershed or drainage basin. Therefore, basin geometry, geology, climate, location, and land use are integral factors that directly or indirectly influence stream or reservoir water quality. Conversely, water quality changes in reservoirs are the result of physical, chemical, and biological loading, generally through runoff and/or stream transport and processing.
The term “reservoir” encompasses herein any natural or artificial body of water encircled by land, such as but not limited to, natural or artificial lakes, ponds, pools and the like. It is noted that there are many differences between natural and artificial lakes. For example, the deepest portion of a natural lake may be located anywhere, but is often near the center, with all portions of the lake bottom sloping toward that maximum depth. By contrast, the deepest portions of artificial reservoirs are almost always near the dam, and the reservoir bottom usually slopes toward the dam. Also, the inlet and outlet of natural lakes are near the surface, whereas an artificial reservoir can release water from any location, ranging from the surface to the deepest portion of the impoundment. Consequently, although the limnological (study of freshwater bodies) processes determining water quality conditions are the same in both cases, the hydrodynamics of artificial reservoirs make their water quality characteristics different than those of natural lakes. From an ecological point of view, an artificial reservoir normally has variable productivity potential levels - high in the early years, low during the following years and then, sometimes, high again during the reservoir's mature stage. By contrast, the natural lake follows a successional pattern from oligotrophy to eutrophy (see below “Trophic Status”).
Reservoirs, especially natural lakes, have been classified using a variety of systems, including physical, chemical, and geomorphological characteristics, and indicator species or species aggregates. Examples:
a. Stratified Versus Unstratified
Reservoirs may or may not stratify, depending on conditions such as depth, wind mixing, and retention time. Under appropriate conditions, the reservoir will form an epilimnion or upper layer, a metalimnion or transitional layer, and a hypolimnion or lower layer. However, if conditions do not allow stratification, the entire reservoir may consist of an epilimnion with an isothermal gradient. The stratified or unstratified condition can dramatically affect water quality conditions of the reservoir and its releases. Releases from an unstratified reservoir, irrespective of the withdrawal level, will generally be warmwater releases; bottom-level withdrawals from a stratified reservoir will be generally coldwater releases. Warm and cold releases are relative to the water temperature of the stream into which the releases are made.
b. Operational Characteristics
Reservoir projects are authorized for a variety of purposes, the most common of which are flood control, navigation, hydroelectric power generation, water supply, fish and wildlife conservation and enhancement, recreation, and low-flow augmentation.
c. Trophic Status
Reservoirs are commonly classified or grouped by trophic or nutrient status. The natural progression of water bodies through time is from an oligotrophic (i.e., low nutrient/low productivity) through a mesotrophic (i.e., intermediate nutrient/intermediate productivity) to a eutrophic (i.e., high nutrient/high productivity) condition. The prefixes “ultra” and “hyper” are sometimes added to oligotrophic and eutrophic, respectively, as additional degrees of trophic status. The tendency toward the eutrophic or nutrient-rich status is common to all impounded waters.
The eutrophication or enrichment process has received considerable study because:
(a) It can be accelerated by nutrient additions through cultural activities (e.g., point-source discharges and nonpoint sources such as agriculture, urbanization, etc.).
(b) Water quality conditions associated with eutrophication may not be desired.
(c) To a certain degree, cultural eutrophication impacts are reversible.
The majority of reservoir water quality conditions relate to the eutrophication process. Certain physical, chemical, and biological factors change during eutrophication.